<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="https://cdn.jsdelivr.net/npm/vue@2.6.14/dist/vue.js"></script>
</head>
<body>
    <div id="app">
        <div id="test" ref="test">{{msg}}</div>
    </div>
    <script>
        new Vue({
            el:"#app",
            data:{
                msg:"good"
            },
            beforeCreate() {
                console.log(this.$refs.test)  // undefined
            },
            created() {
                console.log(this.$refs.test)  // undefined
            },
            beforeMount() {
                console.log(this.$refs.test)  // undefined
            },
            // 只能在mounted这个生命周期函数中获取DOM
            mounted() {
                console.log(this.$refs.test) // <div id="test">good</div>
            }
        })
    </script>
</body>
</html>